I've had passer-bys take group photos with a real camera, no problem. What issues do you run into?
On the iPhone, ~everyone on the planet instinctively knows how to do it.
> "Which button do I press"
The Big One.
> focus
It's automatic. (If I'm handing a stranger my camera.)
> zoom level
This is maybe the hardest one, I guess, … but I do think most people have seen enough TV cop dramas to instinctively know. Or, they can just take the photo at the zoom I've handed them, and it won't be a big deal. Walking forward a few steps is also like zooming.
Sometimes you get a stranger looking through a viewfinder and crouching and you know you've got yourself another hobbyist (or pro) as well :) Those compositions usually are better too!
This happened the last time I gave my camera to someone and I was smacking myself, as I'd put the camera into "LCD screen is viewfinder mode", since then it is closer to a phone for most people. And I'd found someone who knew what a viewfinder was! Of course, it's just black. (The mirror being in the way.)